Method for adjusting a wave plate for an imaging system

ABSTRACT

A compact mounting for a wave plate permits convenient rotational adjustment of the wave plate through an angle of at least one-half revolution to exploit asymmetry in the optical characteristics of the wave plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation and claims the benefit of U.S. patentapplication Ser. No. 09/773,396, filed on Jan. 31, 2001.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for mounting an opticalretarder or wave plate.

A retarder or wave plate is an optical element that changes thepolarization of an incident light wave. In principle, the retardercauses the phase of one of the constituent coherent polarization statesto lag behind the other by a predetermined amount. For example, aquarter-wave plate introduces a relative phase shift of 90 degrees (π/2)between the constituent orthogonal o- and e- components of a light wave.Quarter-wave plates are commonly used to convert elliptically polarizedlight to linear polarized light or transform linear polarized light tocircular polarized light. Another common wave plate is the half-waveplate. The half-wave plate introduces a relative phase shift of 180° (π)to incident polarized light. Half-wave plates are commonly used torotate the polarization vector of linear polarized light or to convertthe sense of circular polarized light.

An exemplary use of a quarter-wave plate is in the construction of aliquid crystal light valve projector such as that disclosed by Schmidtet al. in U.S. Pat. No. 5,576,854. Schmidt et al. disclose that theperceived quality of a projected image can be improved by making the“off” or black state as black as possible. This increases the contrastbetween light and dark areas of the projected image. In a liquid crystallight valve projector, light from a source is projected onto a polarizerwhich reflects S-polarized light to a liquid crystal panel. If a part ofthe liquid crystal light panel is in the “on” state, the liquid crystalsbecome birefringent and convert the S-polarized light to P-polarizedlight which is reflected back to the polarizer. This light passesthrough the polarizer and a lens and then on to a projection screen toproduce a “lighted” condition on the screen. If the light valve is inits “off” state, a mirror behind the liquid crystal layer reflects theS-polarized light back onto the polarizing surface. S-polarized lightstriking the polarizer is reflected from the polarizer in the directionof the light source and away from the viewing screen to produce a“black” screen condition.

However, the polarization process is not perfectly efficient and thegenerally S-polarized light reflected from the polarizer includes asmall portion of light that is not S-polarized with respect to thepolarizer. In other words, the generally S-polarized light reflectedonto the polarizing surface has a component in the P-polarizeddirection. When this P-polarized component is reflected by the liquidcrystal panel to the polarizer, it leaks through the polarizer and ontothe projection screen. As a result, the screen is partially illuminatedwhen the light panel is “off” and the screen is supposed to be dark. Aquarter-wave plate with its fast axis either perpendicular or parallelto the axis of polarization of the polarizing surface is located betweenthe polarizer and the liquid crystal panel. The phase of the P-polarizedcomponent is shifted 180° as the light passes through the quarter-waveplate and then is reflected back through the wave plate by the liquidcrystal panel. As a result the light reflected from the “off” liquidcrystal panel will be effectively 100% S-polarized when it impinges onthe polarizer. Since the light no longer includes a component that canleak through the polarizer the screen will be darker improving thecontrast between light and darker areas of the image.

Wave plates are typically produced from uniaxial materials having asingle optic axis and two indices of refraction. Light entering thematerial is divided into two waves which emerge from the material alongtwo axes. Light polarized along the direction of the axis exhibiting thesmaller index of refraction travels faster and, therefore, this axis istermed the fast axis. The second axis or slow axis exhibits the largerindex of refraction and light polarized along the direction of this axistravels slower. Since a quarter-wave plate produces a phase shift of 90°between perpendicular fast and slow axes, the wave plate has heretoforebeen considered to be substantially optically symmetrical about eitherthe fast or slow axis. In other words, 180 degree rotation of thequarter-wave plate about the normal to the intersection of the fast andslow axes has been expected to produce the same optical effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of an optical section of atypical liquid crystal light valve projector.

FIG. 2 is an elevation view of a wave plate mounting of the presentinvention.

FIG. 3 is a section view of the wave plate mounting of FIG. 2 takenalong line 3—3.

FIG. 4 is an elevation view of a journal box for mounting a wave plate.

FIG. 5 is a section view of the journal box of FIG. 4 taken along line5—5.

DETAILED DESCRIPTION OF THE INVENTION

The present inventor was astonished to observe that the performance ofquarter-wave plates in liquid crystal light valve projectors suggeststhat a quarter-wave plate is not optically symmetrical and that, allother factors being equal, a lighter or darker screen can be produced byrotating the quarter-wave plate to one of two positions one-halfrevolution apart. While the adjustment produces significant improvementin the quality of the projected image, establishing better rotationalposition of the wave-plate is most easily determined after assembly ofthe liquid crystal light valve projector optics. However, the compactnature of the assembly makes rotation of the wave plate problematic. Acompact mounting for a wave plate that permits convenient rotation ofthe wave plate through an angle of at least one-half revolution andlocking of the wave plate in a desired position is desirable.

The optical section of a liquid crystal light valve projector isexemplary of an optical system incorporating an optical retarder or waveplate. The basic optical components of a typical liquid crystal lightvalve projector 20 are illustrated in FIG. 1A polarizing beam splitter22 comprises a pair of prisms 24 and 26 with a polarizing beam splittinglayer 28 interposed between the prisms. The polarizing beam splitter hasfour facets 30, 32, 34, and 36. Two facets 30 and 32 are parallel toeach other and normal to a first optical axis 38. The other two facets34 and 36 of the beam splitter 22 are parallel to each other and normalto a second optical axis 40. A quarter-wave plate 42 is positionedbetween the beam splitter 22 and a liquid crystal panel 44 with its twoparallel faces substantially normal to the second optical axis 40. Thequarter-wave plate has a fast axis 46 (a polarization directionmaximizing the phase velocity) and a slow axis 48 (a polarizationdirection minimizing the phase velocity) which are perpendicular to eachother and, substantially, perpendicular to the second optical axis 40.The fast axis 46 is also arranged either perpendicular or parallel tothe axis of polarization of the polarizing beam splitting layer 28.

Light 52 from a source 50 comprising approximately parallel rays entersthe beam splitter 22 along the first optical axis 38 and is separated atthe splitting layer 28 into a P-polarized component and a S-polarizedcomponent. The P-polarized component 54 passes straight throughsplitting layer 28 in the direction of the first optical axis 38. TheS-polarized component 56 is reflected from the splitting layer 28 alongthe second optical axis 40. The S-polarized light component 56 passesthrough the quarter-wave plate 42 and impinges on the liquid crystalpanel 44.

The liquid crystal panel 44 comprises generally a liquid crystal layer,an array of reflective electrodes, and an array of switching elementsarranged behind the reflective electrodes. The switching elements andreflective electrodes of the arrays correspond to picture elements orpixels of the image to be created on liquid crystal panel 44. The liquidcrystals of a pixel are isotropic when the pixel is “off,” that is,unless a voltage is applied to a switching element corresponding to thepixel. If the pixel is off, the S-polarized light 56 incident on theliquid crystal panel 44 is reflected 62 as S-polarized light by thereflective electrode. However if a voltage is applied to a switchingelement the pixel is turned “on.” When the pixel is “on,” the liquidcrystals of the pixel become birefringent and the S-polarized light 56incident on the panel is converted to P-polarized light. As a result,the polarization state of the light reflected from the reflectiveelectrode for a pixel varies depending upon the whether a voltage isapplied to the switching element for the pixel.

When P-polarized light reflected from an “on” pixel 62, strikes thepolarizing beam splitting surface 28 the light passes through thesurface in the direction of optical axis 40. The light passes throughthe lens 66 and eventually strikes the projection screen (notillustrated). On the other hand, when S-polarized light reflected froman “off” pixel 62 of the liquid crystal panel 44 strikes the polarizingbeam splitting surface 28 of the beam splitter 22 the light is reflectedaway, generally in the direction of the light source 50 along theoptical axis 30. The result is a dark pixel on the projection screen,

However, the polarization of light from the source 50 by the polarizingbeam splitting surface 28 is not perfectly efficient. The light source50 is not a point source and while the light is generally incident onthe polarizing beam splitting surface 28 along the optical axis 38, someof the light strikes the beam splitting surface 28 at other angles. As aresult, the direction of light reflected from an “off” pixel of theliquid crystal panel 44 is not identical to the S-polarization directionof the polarizing beam splitting layer 28. In other words, the lightreflected from an “off” pixel of the liquid crystal panel 44 includeslight having a component polarized in the P-polarization directionrelative to the beam splitting surface 28. If this light strikes thepolarizing beam splitting surface 28, the light with P-polarization willleak through the beam splitting surface 28 and onto the projectionscreen, reducing the contrast between light and dark pixels.

The quarter-wave plate 42 positioned between the beam splitter 22 andthe liquid crystal panel 44 acts as a half-wave plate because the lightfirst passes through the wave plate and then is reflected back throughthe wave plate by the liquid crystal panel 44. Each time the lightpasses through the quarter-wave plate 42 the phase of the P-component isshifted 90°. In other words, the phase of the P-component is shifted180° by two passes through the quarter-wave plate 42 and thepolarization of the light striking polarizing beam splitting surface 28is in the negative P-polarization direction. As a result, the lightstriking the polarizing beam splitting surface 28 is effectively 100%S-polarized and does not include a component that can leak through thepolarizing beam splitting surface 28 to illuminate dark pixels andreduce the contrast in the projected image.

Wave plates are typically constructed of uniaxial materials such asquartz, mica or oriented polymeric plastic. A uniaxial material ischaracterized by a single optic axis and two indices of refraction. Whena monochromatic light wave is incident on a uniaxial material the lightwave is generally divided into two waves which emerge from the materialpolarized along a “fast” axis and a “slow” axis. If the uniaxialmaterial is arranged correctly with respect to the optic axis of theincident light, the minimum index of refraction is exhibited for onepolarization of a plane polarized wave and the maximum index ofrefraction will be exhibited for the second polarization. Lightpolarized along the direction of the axis with the lesser index ofrefraction travels faster and, therefore, the axis is termed the fastaxis. The second axis or slow axis exhibits the larger index ofrefraction and light polarized along the direction of this axis travelsslower. Since a quarter-wave plate produces a relative phase shift of90° for light polarized along the perpendicular fast and slow axes, thewave plate has heretofore been considered to be generally opticallysymmetrical about the fast and slow axes. Rotating a quarter-wave plateone-half revolution about a normal to the intersection of the fast andslow axes was expected produce the same effect on the incident polarizedlight. However, the present inventor observed that the level ofillumination of a dark projection screen is not equal when a quarterwave plate positioned between a liquid crystal light panel and a beamsplitter is rotated one half revolution about the normal to theintersection of the fast 46 and slow axis 48. While the reason for thisphenomenon is not fully understood, rotation of the quarter-wave plateto one of two positions separated by one-half a revolution maximizes thecontrast in a projected image.

The present invention provides a mounting apparatus for a wave platethat permits the wave plate of an assembled liquid crystal light valveprojector to be rotated at least one-half revolution about theintersection of the fast and slow axes to an orientation producing amaximum contrast in a projected image. The wave plate can then be lockedinto the rotational orientation producing the best result. The relativearrangements of components of the liquid crystal light valve projectorare illustrated in FIG. 1. However, the actual components are typicallymuch smaller than illustrated in FIG. 1 and typically arrangedimmediately adjacent to each other. Therefore, if rotational adjustmentis to be conveniently accomplished with an assembled projector, themechanism must be very compact and must facilitate rotation of the waveplate without disassembly of the projector. Referring to FIGS. 2 and 3,the wave plate mounting apparatus 80 of the present invention provides acompact assembly facilitating convenient rotation of the wave plate. Thewave plate mounting apparatus comprises generally a journal box 82adapted to support a wave plate 42 for rotation. The journal box 82 isadapted for attachment to a supporting frame (not illustrated) of theliquid crystal light valve projector 20 by riveting, welding, orotherwise, The wave plate 42 is supported for rotation in the journalbox 82 by a frame 84. While the illustrated frame 84 is an annulussurrounding a circular wave plate 42, a wave plate of arbitrary shapecould be retained in an appropriate frame. Referring to FIGS. 4 and 5,the journal box 82 includes portions defining a stepped bore 85. Aportion of the stepped bore 85 forms an aperture 86 which is alignedgenerally with the optical axis 40 of the projector permitting light topass between the polarizing beam splitting surface 28 and the liquidcrystal panel 44. A stepped portion of the bore 85 forms concentricstepped surfaces 87 and 89 to support the frame 84 for rotation about anormal to the intersection (coincident with the optical axis 44) of thefast and slow axes of wave plate.

To facilitate rotation of the wave plate in the close confines of theliquid light valve projector assembly, a bendable member or tape 88having a first end affixed to the frame 84 is wound around the peripheryof the frame 84. The first end of the tape 88 is affixed to the frame 84by either inserting an end portion into a slot (not illustrated) cut inthe frame, by welding, or other means of attachment. The tape 88 isrestrained on the periphery of the frame 84 by a groove formed a byshoulder portion 97 of the frame projecting radially from the frame 84at one face. The second end of the tape 88 is fed through a groove 90 inthe journal box 82. To rotate the wave plate 42, the second end of thetape 88 is pulled or pushed, altering the length of the tape 88proximate to the periphery of the frame 84. After adjustment, therotational orientation of the wave plate producing the best results canbe locked into the assembly with a set screw 92 or other lock deviceanchoring position of the second end of the tape 88 relative to theframe 84 (or any other technique for restricting rotational movement ofthe wave plate). With the mounting of the present invention, a waveplate can be conveniently rotated in an assembled projector to theorientation producing the better results and locked into position foruse. It is noted that the wave plate is preferably rotatable about atleast 90 degrees or more, more preferably 180 degrees or more, and bestif 360 degrees or more.

All the references cited herein are incorporated by reference.

The terms and expressions that have been employed in the foregoingspecification are used as terms of description and not of limitation,and there is no intention, in the use of such terms and expressions, ofexcluding equivalents of the features shown and described or portionsthereof, it being recognized that the scope of the invention is definedand limited only by the claims that follow.

1. A method of adjusting a wave plate for an imaging system comprising:(a) providing a beam splitter; (b) providing an imaging device; (c)providing a supporting structure including said wave plate positionedbetween said beam splitter and said imaging device; and (d) mechanicallyadjusting the imaging system by rotating said wave plate about an axisperpendicular to said wave plate located within the periphery of saidwave plate over a range in excess of 90 degrees, wherein said wave plateis subsequently maintained in the same rotational position irrespectiveof a change in a polarization state of light passing through said waveplate.
 2. The method of claim 1 further comprising adjusting the imagingsystem by rotating said wave plate over a range in excess of 180degrees.
 3. The method of claim 2 wherein said adjusting is to align atleast one optical axis of said wave plate with respect to said imagingdevice.
 4. The method of claim 1 further comprising adjusting theimaging system by rotating said wave plate over a range in excess of 360degrees.
 5. The method of claim 4 wherein said adjusting is to align atleast one optical axis of said wave plate with respect to said imagingdevice.
 6. The method of claim 5 further comprising selecting anoptically preferable orientation of said wave plate from twoorientations approximately 180 degrees apart.
 7. The method of claim 1wherein said adjusting is to align at least one optical axis of saidwave plate with respect to said imaging device.
 8. A method ofincreasing contrast in an image displayed by an imaging system, saidmethod comprising the steps of: (a) providing a beam splitter; (b)providing an imaging device; (c) providing a wave plate supportingstructure; (d) rotatably supporting a wave plate in said wave platesupporting structure, said wave plate positioned between said beamsplitter and said imaging device; and (e) mechanically increasingcontrast in an image by rotating said wave plate about an axisperpendicular to said wave plate located within the periphery of saidwave plate over a range in excess of 90 degrees, wherein said wavesubsequently maintained in the same rotational position irrespective ofa change in a polarization state of light passing through said waveplate.
 9. The method of claim 8 wherein said adjusting is to align atleast one optical axis of said wave plate with respect to said imagingdevice.
 10. The method of claim 8 further comprising adjusting contrastin said image by rotating said wave plate over a range in excess of 180degrees.
 11. The method of claim 10 wherein said adjusting is to alignat least one optical axis of said wave plate with respect to saidimaging device.
 12. The method of claim 8 further comprising adjustingcontrast in said image by rotating said wave plate over a range inexcess of 360 degrees.
 13. The method of claim 12 wherein said adjustingis to align at least one optical axis of said wave plate with respect tosaid imaging device.
 14. The method of claim 8 further comprising thestep of selecting an optically preferable orientation of said wave platefrom two orientations approximately 180 degrees apart.
 15. A method ofincreasing contrast in an image displayed by an imaging system, saidmethod comprising the steps of: (a) providing a beam splitter; (b)providing an imaging device; (c) providing a wave plate supportingstructure; (d) rotatably supporting a wave plate in said wave platesupporting structure, said wave plate positioned in a first rotationalorientation between said beam splitter and said imaging device; and (e)mechanically increasing contrast in an image by rotating said wave plateto a second rotational orientation about an axis perpendicular to saidwave plate located within the periphery of said wave plate over a rangein excess of 90 degrees; and (f) locking said wave plate into saidsecond orientation.
 16. The method of claim 15 wherein said adjusting isto align at least one optical axis of said wave plate with respect tosaid imaging device.
 17. The method of claim 15 further comprisingadjusting contrast in said image by rotating said wave plate over arange in excess of 180 degrees.
 18. The method of claim 17 wherein saidadjusting is to align at least one optical axis of said wave plate withrespect to said imaging device.
 19. The method of claim 15 furthercomprising adjusting contrast in said image by rotating said wave plateover a range in excess of 360 degrees.
 20. The method of claim 19wherein said adjusting is to align at least one optical axis of saidwave plate with respect to said imaging device.